California Code, Civil Code - CIV § 1954.207

(a) At the beginning of a tenancy, a submeter shall be read after the tenant takes possession. If the regular reading occurs less than five days prior to the tenant taking possession, that reading may be substituted to establish usage. If the submeter is manually read, the first bill may be estimated based on the rate established in subdivision (b) of Section 1954.212.

(b) For a water-service bill at the end of a tenancy, the submeter shall be read within five days, if possible. If the submeter cannot be read within five days at the end of a tenancy, the bill amount for the final month shall be based on the bill amount for the previous month.

(c) The landlord may, at his or her discretion, deduct an unpaid water service bill from the security deposit during or upon termination of a tenancy, if the last water service bill showing the amount due is attached to the documentation required by Section 1950.5.